Bonding nonclassical

Figure 4.6 Interaction diagrams of a carbocation with a C=C pi bond and a C-H sigma bond. Nonclassical Carbocations...

The Brown-Winstein nonclassical ion controversy can be summed up as differing explanations of the same experimental facts (which were obtained repeatedly and have not been questioned) of the observed significantly higher rate of the hydrolysis of the 1-exo over the 2-endo-norbornyl esters. As suggested by Winstein, the reason for this is participation of the Ci-Q single bond leading to delocalization in the bridged nonclassical ion. In contrast. Brown maintained that the... 

Nonclassical ions, a term first used by John Roberts (an outstanding Caltech chemist and pioneer in the field), were defined by Paul Bartlett of Harvard as containing too few electrons to allow a pair for each bond i.e., they must contain delocalized (T-electrons. This is where the question stood in the early 1960s. The structure of the intermediate 2-norbornyl ion could only be suggested indirectly from rate (kinetic) data and observation of stereochemistry no direct observation or structural study was possible at the time. 

Penta- (or higher) coordinate ( nonclassical carbonium ions contain five or (higher) coordinate carbon atoms. They cannot be described by two-electron two-center single bonds alone but also neces-... 

Some characteristic bonding natures in typical nonclassical ions are the following. 

Both acetolyses were considered to proceed by way of a rate-determining formation of a carbocation. The rate of ionization of the ewdo-brosylate was considered normal, because its reactivity was comparable to that of cyclohexyl brosylate. Elaborating on a suggestion made earlier concerning rearrangement of camphene Itydrochloride, Winstein proposed that ionization of the ero-brosylate was assisted by the C(l)—C(6) bonding electrons and led directly to the formation of a nonclassical ion as an intermediate. 

Attack ty acetate at C-1 of C-2 would be equally likely and would result in equal amounts of the enantiomeric acetates. The acetate ester would be exo because reaction must occur from the direction opposite the bridging interaction. The nonclassical ion can be formed directly only from the exo-brosylate because it has the proper anti relationship between the C(l)—C(6) bond and the leaving group. The bridged ion can be formed from the endo-brosylate only after an unassisted ionization. This would explain the rate difference between the exo and endo isomers. 

The description of the nonclassical norbomyl cation developed by Wnstein implies that the nonclassical ion is stabilized, relative to a secondary ion, by C—C a bond delocalization. H. C. Brown of Purdue University put forward an alternative interpreta-tioiL He argued that all the available data were consistent with describing the intermediate as a rapidly equilibrating classical ion. The 1,2-shift that interconverts the two ions was presumed to be rapid relative to capture of the nucleophile. Such a rapid rearrangement would account for the isolation of racemic product, and Brown proposed that die rapid migration would lead to preferential approach of the nucleophile fiom the exo direction. 

Many other cations besides the norbomyl cation have nonclassical structures. Scheme 5.5 shows some examples which have been characterized by structural studies or by evidence derived from solvolysis reactions. To assist in interpretation of the nonclassical stmctures, the bond representing the bridging electron pair is darkened in a corresponding classical stmcture. Not surprisingly, the borderline between classical stmctures and nonclassical stmctures is blurred. There are two fundamental factors... 

Rauscher 42 calculated that the protonation of a double bond (Eq. (2)) leads to nonclassical, that means H-bridged, cations if the double bond is substituted symmetrically (Rj = R and Rj = R ). 

In discussing nonclassical carbocations we must be careful to make the distinction between neighboring-group participation and the existence of nonclassical carbocations. ° If a nonclassical carbocation exists in any reaction, then an ion with electron delocalization, as shown in the above examples, is a discrete reaction intermediate. If a carbon-carbon double or single bond participates in the departure of the leaving group to form a carbocation, it may be that a nonclassical carbocation is involved, but there is no necessary relation. In any particular case, either or both of these possibilities can be taking place. 

In the following pages, we consider some of the evidence bearing on the questions of the participation of 7t and a bonds and on the existence of nonclassical... 

Furthermore, 48 solvolyzed 350 times faster than its endo isomer 51. Similar high exo/endo rate ratios have been found in many other [2.2.1] systems. These two results—(1) that solvolysis of an optically active exo isomer gave only racemic exo isomers and (2) the high exo/endo rate ratio—were interpreted by Winstein and Trifan as indicating that the 1,6 bond assists in the departure of the leaving group and that a nonclassical intermediate (52) is involved. They reasoned that solvolysis of the endo isomer 51 is not assisted by the 1,6 bond because it is not in a favorable position for backside attack, and that consequently solvolysis of 51 takes... 

The concepts of ct participation and the nonclassical ion 52 have been challenged by Brown, who suggested that the two results can also be explained by postulating that 48 solvolyzes without partieipation of the 1,6 bond to give the classical ion 53, which is in rapid equilibrium with 54. This... 

G. A. Papoian, R. Hoffmann, Hypervalent bonding in one, two and three dimensions extending the Zintl-Klemm concept to nonclassical electron-rich networks. Angew. Chem. Int. Ed. 39 (2000) 2408. 

The complex OsHCl(CO)(P Pr3)2 reacts with HX (X = H, SiEt3, Cl) molecules to give derivatives of the type OsXCl(r)2-H2)(CO)(P Pr3)2 (X = H, SiEt3, Cl), where the hydrogen atoms bonded to the osmium atom undergo nonclassical interaction (Scheme 16). 

Recently, some attempts were nndertaken to uncover the intimate mechanism of cation-radical deprotonation. Thns, the reaction of the 9-methyl-lO-phenylanthracene cation-radical with 2,6-Intidine (a base) was stndied (Ln et al. 2001). The reaction proceeds through two steps that involve the intermediary formation of a cation-radical/base complex before unimolecular proton transfer and separation of prodncts. Based on the value of the kinetic isotope effect observed, it was concluded that extensive proton tnnneling is involved in the proton-transfer reaction. The assumed structure of the intermediate complex involves n bonding between the unshared electron pair on nitrogen of the Intidine base with the electron-deficient n system of the cation-radical. Nonclassical cation-radicals wonld also be interesting reactants for snch a reaction. The cation-radical of the nonclassical natnre are known see Ikeda et al. (2005) and references cited therein. 

---